We are aware of the issue and we are working to resolve for the accounts affected.

For users that may have joined this thread after my posts (they are at the top), you can still check your mail through your computer's Web browser. If you're having trouble on your mobile device or tablet, you also use the device's browser or for iOS and Android, download the Gmail application.

I know this isn't your first choice, but it will allow you to continue accessing your messages in the meantime.

Google has also posted a work-around on its Gmail "known problems" Web page, under the heading "Server error when retrieving mail via IMAP," under the "POP, IMAP, and Sync" section: "[We're] aware of an issue where users are receiving a server error when using IMAP. We are currently looking into this issue. In the meantime, you can log in to Gmail through your phone or tablet's browser. For iOS users, you can also download the Gmail application as an alternative in the interim."